In fuel cells, a fuel and an oxidant are supplied to two electrically-connected electrodes to electrochemically oxidize the fuel, thereby converting chemical energy directly to electrical energy. Unlike thermal power generation, fuel cells are not limited by the Carnot cycle; therefore, they show high energy conversion efficiency. A fuel cell generally comprise a stack of fuel cells, each having an electrolyte layer sandwiched by a pair of electrodes, that is, a membrane-electrode assembly as the basic structure.
Supported platinum and platinum alloy materials have been used as electrocatalysts for cathode and anode of fuel cells. However, such an amount of platinum as is necessary in the new cutting-edge electrocatalysts, is still too expensive to realize commercial mass production of fuel cells. Since noble metal unit cost has a large influence on catalyst price, a further increase in the activity per unit mass of noble metal is desired.
While platinum ions are eluted under a high potential environment, platinum ions are deposited under a low potential environment. Therefore, agglomeration of platinum particles occurs after high potential discharge and low potential discharge are repeated alternately. Such agglomeration of platinum particles causes a decrease in effective electrode area and contributes to a decrease in battery performance.
Previous studies aiming at increasing both catalytic activity and durability, include a study of electrocatalyst having a so-called core-shell structure. An electrocatalyst for fuel cells is disclosed in Patent Literature 1, in which an electroconductive carrier supports particles comprising a noble metal such as platinum and having a core-shell structure such that the core comprises at least a noble metal, such as platinum and the shell comprises a noble metal oxide and is formed around the core.